Honeycomb structure

ABSTRACT

A honeycomb structure including a honeycomb portion having porous partition walls extending from an inflow end face to an outflow end face to define cells forming through channels, an outermost peripheral wall, and a pair of electrode portions disposed on a side surface of the honeycomb portion. The electrode portions are formed in a strip shape extending in a direction of the cells. In a cross section orthogonal to the extending direction, one electrode portion of the pair of electrode portions is disposed on a side opposed to the other electrode portion across a center of the honeycomb structure portion. The honeycomb structure portion includes end regions near the pair of electrode portions and a central region excluding the end regions. An average electric resistivity A of a material forming the end regions is lower than an average electric resistivity B of a material forming the central region.

This application claims the benefit under 35 USC § 119(a)-(d) ofJapanese Application No. 2018-063062 filed Mar. 28, 2018, the entiretyof which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a honeycomb structure.

BACKGROUND OF THE INVENTION

Conventionally, a member in which a catalyst is supported on a honeycombstructure made of cordierite or silicon carbide is used for treatment ofharmful substances in exhaust gases discharged from motor vehicleengines (see, Patent Document 1). Such a honeycomb structure generallyhas a pillar shaped honeycomb structure that includes partition wallsdefining a plurality of cells extending from one end face to the otherend face to form flow paths for an exhaust gas.

For the treatment of the exhaust gas with the catalyst supported on thehoneycomb structure, a temperature of the catalyst is required for beingincreased to a predetermined temperature. However, as the engine isstarted, the catalyst temperature is lower, conventionally causing aproblem that the exhaust gas is not sufficiently purified. Therefore, asystem called an electric heating catalyst (EHC) has been developed. Inthe system, electrodes are disposed on a honeycomb structure made ofconductive ceramics and the honeycomb structure itself generates heat byelectric conduction, whereby the temperature of the catalyst supportedon the honeycomb structure is increased to an activation temperaturebefore or during starting of the engine.

Patent Document 1 proposes a honeycomb structure which is a catalystsupport and also functions as a heater by applying a voltage, and whichcan suppress a bias of a temperature distribution when a voltage isapplied. More particularly, it proposes that the bias of the temperaturedistribution when the voltage is applied is suppressed by disposing apair of electrode portions in the form of strip on two positions on aside surface of the pillar shaped honeycomb structure in an extendingdirection of a cell of the honeycomb structure, and disposing oneelectrode portion of the pair of electrode portions so as to be opposedto the other electrode portion of the pair of electrode portions acrossa center of the honeycomb structure, in a cross section orthogonal tothe extending direction of the cell.

Since a portion having a honeycomb structure (i.e., a portion serving asa support for a catalyst; hereinafter referred to as a “honeycombstructure portion”) has usually higher electric resistance than that ofelectrode portions, current from a terminal connected to the electrodeportion tends to be spread out in the electrode portion, before flowingthrough the honeycomb structure portion. However, when the electricalresistance inside the honeycomb structure is uniform, there is a problemthat a large amount of current flows near an end portion of theelectrode portion where a distance passing through the honeycombstructure portion is shorter, so that a heat generation distribution ofthe honeycomb structure portion is biased to generate variations in theheating of the catalyst.

To solve the problem, Patent Document 2 discloses that the thickness ofthe partition wall of the support is set such that the electricresistance of all the current paths between the terminals is equal tothat of the hollow case forming the exterior (paragraph [0009] of PatentDocument 2).

Further, to solve the above problem, Patent Document 3 discloses ahoneycomb structure in which an electric resistivity of a material thatserves as a fluid flow path and forms an outer peripheral region islower than an electric resistivity of a material forming a centralregion (paragraph [0013] of Patent Document 3).

CITATION LIST Patent Literatures

-   -   Patent Document 1: WO 2013/146955 A1    -   Patent Document 2: Japanese Patent Application Publication No.        2011-99405 A    -   Patent Document 3: Japanese Patent Application Publication No.        2014-198321 A

SUMMARY OF THE INVENTION

Patent Document 2 is intended to heat uniformly the honeycomb structureportion by setting the thickness of each partition wall of the honeycombstructure portion so as to satisfy predetermined conditions. However,there is a problem that the setting of the thickness of each partitionwall of the honeycomb structure portion in accordance with the currentresults in positions where mechanical strength is partially lower, andthe strength as the catalyst support is decreased.

Further, Patent Document 3 is intended to heat uniformly the honeycombstructure portion by setting the electric resistivity of the honeycombstructure portion to be lower in the outer peripheral region than in thecentral region. However, there is a problem that if the electricresistivity of the outer peripheral region is lower, current flowsthrough the outer peripheral portion, so that it is difficult to heatthe central region.

The present invention has been made in view of the above problems. Anobject of the present invention is to provide a honeycomb structurecapable of more uniformly generating heat (without bias of heatgeneration distribution) than the prior arts.

As a result of intensive studies, the present inventors have found thatthe above problems can be solved by controlling the distribution ofelectric resistivity in end regions and a central region of thehoneycomb structure portion. Thus, the present invention is specified asfollows:

(1) A honeycomb structure, comprising:

a pillar shaped honeycomb structure portion having:

-   -   porous partition walls extending through the honeycomb structure        from an inflow end face to an outflow end face to define a        plurality of cells forming a through channel;    -   an outer peripheral wall located at the outermost periphery; and

a pair of electrode portions disposed on a side surface of the honeycombstructure portion;

wherein each of the pair of electrode portions is formed in a stripshape extending in an extending direction of the cells of the honeycombstructure portion;

wherein, in a cross section orthogonal to the extending direction of thecells, one electrode portion of the pair of electrode portions isdisposed on a side opposed to the other electrode portion across acenter of the honeycomb structure portion;

wherein the honeycomb structure portion consists:

-   -   end regions near the pair of electrode portions; and    -   a central region that is a center region excluding the end        regions; and

wherein an average electric resistivity A of a material forming the endregions is lower than an average electric resistivity B of a materialforming the central region.

(2) The honeycomb structure according to (1), wherein the A and the Bsatisfy a relationship: ⅕≤A/B≤⅘.

(3) The honeycomb structure according to (1) or (2), wherein thehoneycomb structure portion is mainly based on a silicon-silicon carbidecomposite material or silicon carbide.

(4) The honeycomb structure according to any one of (1) to (3), whereinthe honeycomb structure portion has an electric resistivity of from 0.1to 100 Ωcm and the electrode portions have an electric resistivity offrom 0.001 to 1.0 Ωcm.

(5) The honeycomb structure according to any one of (1) to (4), whereineach of the electrode portions has a central angle of from 60 to 120°.

According to the present invention, it is possible to provide ahoneycomb structure capable of more uniformly generating heat than theprior arts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of a honeycomb structure portion inthe present invention.

FIG. 2 is a cross-sectional view of a honeycomb structure according toan embodiment of the present invention.

FIG. 3 is a view showing a central angle of each electrode portion in anembodiment of the present invention.

FIG. 4 is a view showing end regions and a central region in anembodiment of the present invention.

FIG. 5 is a view showing an outline of current paths in an embodiment ofthe present invention.

FIG. 6 is a view showing measurement points for temperatures andelectric resistivity of end regions and a central region in anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of a support for an electrically heating typecatalyst according to the present invention will be described withreference to the drawings. However, the present invention is not limitedto the embodiments, and various changes, modifications, and improvementsmay be added without departing from the scope of the present invention,based on knowledge of those skilled in the art.

(1. Honeycomb Structure Portion)

FIG. 1 is a view showing an example of a honeycomb structure portion ofa honeycomb structure 100 in the present embodiment. The honeycombstructure portion 10 includes: porous partition walls 11 extendingthrough the honeycomb structure 100 from an inflow end face to anoutflow end face to define a plurality of cells 12 forming a throughchannel and an outer peripheral wall located at the outermost periphery.The number, arrangement, shape and the like of the cells 12, as well asthe thickness of each partition wall 11, and the like, are not limitedand may be optionally designed as required.

A material of the honeycomb structure portion 10 is not particularlylimited as long as it has conductivity, and metals, ceramics and thelike may be used. In particular, from the viewpoint of compatibility ofheat resistance and conductivity, preferably, the material of thehoneycomb structure portion 10 is mainly based on a silicon-siliconcarbide composite material or silicon carbide, and more preferably, itis a silicon-silicon carbide composite material or silicon carbide.Tantalum silicide (TaSi₂) and chromium silicide (CrSi₂) may also beadded to lower the electric resistivity of the honeycomb structure. Thephrase “the honeycomb structure portion 10 is mainly based on asilicon-silicon carbide composite” means that the honeycomb structureportion 10 contains 90% by mass or more of the silicon-silicon carbidecomposite material (total mass) based on the entire honeycomb structureportion. Here, for the silicon-silicon carbide composite material, itcontains silicon carbide particles as an aggregate and silicon as abonding material for bonding the silicon carbide particles, and aplurality of silicon carbide particles are bonded by silicon so as toform pores between the silicon carbide particles. The phrase “thehoneycomb structure portion 10 is mainly based on silicon carbide” meansthat the honeycomb structure portion 10 contains 90% by mass or more ofsilicon carbide (total mass) based on the entire honeycomb structureportion.

The electric resistivity of the honeycomb structure portion 10 may beset as needed depending on voltage to be applied, including, but notparticularly limited to, from 0.01 to 100 Ω·cm, for example. For ahigher voltage of 64 V or more, it may be from 2 to 200 Ω·cm, andtypically from 5 to 100 Ω·cm. Further, for a lower voltage of less than64 V, it may be from 0.001 to 2 Ω·cm, and typically from 0.001 to 1Ω·cm, and more typically from 0.01 to 1 Ω·Cm. As used herein, theelectric resistivity of the honeycomb structure portion 10 refers to anelectric resistivity when measured by a four-terminal method with amultimeter.

In addition, the distribution of the electric resistivity of thehoneycomb structure 10 in end regions and a central region will bedescribed below.

Each partition wall 11 of the honeycomb structure portion 10 preferablyhas a porosity of from 35 to 60%, and more preferably from 35 to 45%.The porosity of less than 35% may result in larger deformation duringfiring. The porosity of more than 60% may result in decreased strengthof the honeycomb structure portion. The porosity is a value measured bya mercury porosimeter.

Each partition wall 11 of the honeycomb structure portion 10 preferablyhas an average pore size of from 2 to 15 μm, and more preferably from 4to 8 μm. The average pore diameter of less than 2 μm may result inexcessively higher electric resistivity. The average pore diameter ofmore than 15 μm may result in excessively lower electric resistivity.The average pore size is a value measured by a mercury porosimeter.

The shape of each cell 12 in a cross section of each cell orthogonal toa flow path direction is not limited, but it may preferably be a square,a hexagon, an octagon, or a combination thereof. Among these, the squareand hexagonal shapes are preferable. Such a cell shape leads to adecreased pressure loss when an exhaust gas flows through the honeycombstructure portion 10, and improved purification performance of thecatalyst.

The outer shape of the honeycomb structure portion 10 is notparticularly limited as long as it is a pillar shape. Further, for thesize of the honeycomb structure portion 10, the honeycomb structureportion preferably has an area of bottom surfaces of from 2000 to 20000mm², and more preferably from 4000 to 10000 mm², in terms of increasingheat resistance (preventing cracks generated in a circumferentialdirection of the outer peripheral side wall). Further, an axial lengthof the honeycomb structure portion 10 is preferably from 50 to 200 mm,and more preferably from 75 to 150 mm, in terms of increasing the heatresistance (preventing cracks generated in a direction parallel to acentral axis direction on the outer peripheral side wall).

The outer peripheral wall 3 forming the outermost periphery of thehoneycomb structure portion 10 of the honeycomb structure 100 accordingto the present embodiment preferably has a thickness of from 0.1 to 2mm. If it is less than 0.1 mm, the strength of the honeycomb structure100 may be decreased. If it is more than 2 mm, the areas of thepartition walls supporting the catalyst may be decreased.

Further, the honeycomb structure portion 10 can be used as a catalystsupport by supporting a catalyst on the honeycomb structure portion 10.

In the present embodiment, when discussing the honeycomb structure 10 bydividing it into end regions near a pair of electrode portions asdescribed below and a central region that is a center region excludingthe end regions, it is important that an average electric resistivity Aof a material forming the end regions is lower than an average electricresistivity B of a material forming the central region.

Here, the end regions and the central region are defined as follows.First, in a cross section orthogonal to the extending direction of thecell of the honeycomb structure portion 10, a straight line L is drawnso as to connect center points of a pair of electrode portions 21, 21,which is a circumferential length of the honeycomb structure portion 10.As described below, the pair of electrode portions 21, 21 are disposedon sides of the honeycomb structure portion 10, which are opposite toeach other across a center O of the honeycomb structure portion 10, sothat the straight line L passes through the center O of the honeycombstructure portion 10. Therefore, the length of the straight line L is adiameter of the honeycomb structure portion 10. Straight lines are thendrawn so as to form a central angle of 5° on each of left and rightsides using the straight line L as a center. Therefore, both thestraight lines define regions of the central angle 10° around thestraight line L. Among the regions, each region up to the length of ⅕from the outer peripheral wall of the honeycomb structure 10 is definedas the end region.

The portion excluding the end regions is defined as the central region.

When the A is lower than the B, a current easily flows through the endregions of the honeycomb structure portion 10 (see FIG. 5), therebyallowing effective suppression of the problem that a large amount ofcurrent flows near the end portions of the electrode portion, the heatgeneration distribution of the honeycomb structure portion is biased togenerate variations in the heating of the catalyst, as described above.Therefore, the vicinity of the center O of the honeycomb structureportion 10 is heated as compared with a case where a current flows onlyin the outer peripheral region of the honeycomb structure portion 10, sothat the entire honeycomb structure portion 10 is heated by current heator thermal conduction of the partition walls, which makes thetemperature distribution more uniform.

The preferred ranges of A and B are the same as described above.

Further, in terms of strengthening the current path passing through notonly the outer periphery of the honeycomb structure 10 but also thecenter, the A and the B preferably satisfy the relationship: ⅕≤A/B≤⅘.

(2. Electrode Portion)

As shown in FIG. 2, the honeycomb structure portion 10 according to thepresent embodiment include a pair of electrode portions 21 provided incontact with the outer surface of the outer peripheral side wall and soas to be opposed to each other across the center O of the honeycombstructure portion 10. Each of the pair of electrode portions 21, 21 isformed in a “strip shape” extending in the extending direction of thecell 12 of the honeycomb structure portion 10. Thus, in the honeycombstructure 100 of the present embodiment, each electrode portion 21 isformed in a strip shape, the longitudinal direction of each electrodeportion 21 is the extending direction of the cell 12 of the honeycombstructure portion 10, and the pair of electrode portions 21, 21 arearranged to be opposed to each other across the center O of thehoneycomb structure portion 10.

Further, in the cross section orthogonal to the extending direction ofthe cell 12, a central angle α of each of the electrode portions 21, 21is preferably from 60 to 120°. Furthermore, in the cross sectionorthogonal to the extending direction of the cell 12, an upper limit ofthe central angle α of each of the electrode portions 21, 21 ispreferably 110, and more preferably 100. In the cross section orthogonalto the extending direction of the cell 12, a lower limit of the centralangle α of each of the electrode portions 21, 21 is preferably 70, andmore preferably 80. Further, the central angle α of one electrodeportion 21 is preferably from 0.8 to 1.2 times larger than the centralangle α of the other electrode portion 21, and more preferably 1.0 times(the same size). This can allow suppression of the bias of currentflowing through the outer periphery and the central region of thehoneycomb structure portion when a voltage is applied between the pairof electrode portions 21, 21. In each of the outer periphery and thecentral region of the honeycomb structure portion, any bias of heatgeneration can be suppressed.

As used herein, the central angle α refers to an angle formed bystraight lines connecting both end portions of the electrode portions 21and the center O of the honeycomb structure portion, in the crosssection orthogonal to the extending direction of the cell 12 (see FIG.3). In FIG. 3, the central angles α of the pair of electrode portions 21are the same.

In the honeycomb structure 100 according to the present embodiment, theelectric resistivity of the electrode portions 21 is preferably lowerthan the electric resistivity of the outer peripheral wall of thehoneycomb structure portion 10. Further, the electric resistivity of theelectrode portions 21 is more preferably from 0.1 to 10%, andparticularly preferably from 0.5 to 5%, of the electric resistivity ofthe outer peripheral wall of the honeycomb structure portion 10. If itis lower than 0.1%, an amount of current flowing to the “end portions ofthe electrode portion” within the electrode portion 21 will be increasedwhen a voltage is applied to the electrode portions 21, so that thecurrent flowing through the honeycomb structure portion 10 may be easilybiased. In addition, it may be difficult for the honeycomb structure 10to generate heat uniformly. If it is higher than 10%, an amount ofcurrent spreading in the electrode portions 21 is decreased when avoltage is applied to the electrode portions 21, and the current flowingthrough the honeycomb structure portion 10 may be easily biased. Inaddition, it may be difficult for the honeycomb structure 10 to generateheat uniformly.

Each electrode portion 21 preferably has a thickness of from 0.01 to 5mm, and more preferably from 0.01 to 3 mm. The thickness in such a rangecan provide contribution to uniform heat generation of the honeycombstructure portion. If the thickness of each electrode portion 21 is lessthan 0.01 mm, the electric resistivity will be increased and uniformheat generation may not be possible. If the thickness of each electrodeportion 21 is more than 5 mm, breakage may occur during canning.

As shown in FIG. 1, in the honeycomb structure 100 according to thepresent embodiment, each of the electrode portions 21, 21 extends in theextending direction of the cell 12 of the honeycomb structure portion 10and is formed in a strip shape “extending between both end portions(both end faces 13, 14)”. Thus, in the honeycomb structure 100 accordingto the present embodiment, the pair of electrode portions 21, 21 aredisposed so as to extend between both end portions of the honeycombstructure portion 10. This can allow more effective suppression of thebias of the current in the axial direction of the honeycomb structureportion (that is, the extending direction of the cell 12) when a voltageis applied between the pair of electrode portions 21, 21. As usedherein, the phrase “electrode portion 21 is formed (disposed) betweenboth end portions of the honeycomb structure portion 10” has thefollowing meaning: one end portion of the electrode portion 21 is incontact with one end portion (one end face) of the honeycomb structureportion 10 and the other end portion of the electrode portion 21 is incontact with the other end portion (the other end face) of the honeycombstructure portion 10.

On the other hand, a preferable embodiment is also a state where atleast one end portion of each electrode portion 21 in “the extendingdirection of the cell 12 of the honeycomb structure portion 10” is notin contact with the end portion (end face) of the honeycomb structureportion 10. This can improve thermal shock resistance of the honeycombstructure.

In the honeycomb structure 100 of the present embodiment, each electrodeportion 21 is formed in a shape such that a planar rectangular member iscurved along an outer periphery of a pillar shape, for example as shownin FIGS. 1 to 3. Here, a shape when the curved electrode portion 21 isdeformed into a non-curved planar member will be referred to as a“planar shape” of the electrode portion 21. The “planar shape” of theelectrode portion 21 shown in FIGS. 1 to 3 will be a rectangle. An“outer peripheral shape of the electrode portion” as used herein means“an outer peripheral shape in the planar shape of the electrodeportion”.

In the honeycomb structure 100 according to the present embodiment, theouter peripheral shape of the strip-shaped electrode portion may be ashape in which each of rectangular corner portions are formed in acurved shape. Such a shape allows improvement of the thermal shockresistance of the honeycomb structure. A preferable embodiment is thatthe outer periphery of the strip-shaped electrode portion has a shape inwhich the rectangular corner portions are linearly chamfered. Such ashape can allow improvement of the thermal shock resistance of thehoneycomb structure.

In the honeycomb structure 100 according to the present embodiment, thelength of the current path is preferably 1.6 times or less the diameterof the honeycomb structure portion, in the cross section orthogonal tothe extending direction of the cell. If it is more than 1.6 times,energy may be unnecessarily consumed. As used herein, the “current path”refers to a path through which a current flows. The “length of thecurrent path” refers to a length 0.5 times the length of the “outerperiphery” through which current flows, in the “cross section orthogonalto the extending direction of the cell” of the honeycomb structure. Thismeans the maximum length of the “flow paths through which current flows”in the “cross section orthogonal to the extending direction of the cell”of the honeycomb structure. The “length of the current path” is a valuemeasured along surfaces within irregularities or a slit when theirregularities are formed on the outer periphery or the slit opening tothe outer periphery are formed in the honeycomb structure portion.Therefore, for example, when the slit opening to the outer periphery isformed in the honeycomb structure portion, “the length of the currentpath” will be longer by a length approximately two times the depth ofthe slit.

The electric resistivity of the electrode portions 21 is preferably from0.01 to 1.0 Ωcm. By such a range of the electric resistivity of theelectrode portion 21, the pair of electrode portions 21, 21 effectivelyact as electrodes in a pipe through which an exhaust gas at an elevatedtemperature flows. If the electric resistivity of the electrode portion21 is less than 0.01 Ωcm, the temperature of the honeycomb portion nearboth ends of the electrode portion 21 will tend to rise, in the crosssection orthogonal to the extending direction of the cell. If theelectric resistivity of the electrode portion 21 is more than 1.0 Ωcm,current will hardly flow, so that it may be difficult to play a role asan electrode. The electric resistivity of each electrode portion is avalue at room temperature (25° C.).

Each electrode portion 21 preferably has a porosity of from 30 to 60%,and more preferably 30 to 55%. The porosity of each electrode portion 21in such a range can provide a suitable electric resistivity. If theporosity of the electrode portion 21 is less than 30%, deformation willoccur during the production. If the porosity of each electrode portion21 is more than 60%, the electric resistivity may be excessivelyincreased. The porosity is a value measured with a mercury porosimeter.

Each electrode portion 21 preferably has an average pore diameter offrom 5 to 45 μm, and more preferably 7 to 40 μm. The average porediameter of each electrode portion 21 in such a range can provide asuitable electric resistivity. If the average pore diameter of eachelectrode portion 21 is less than 5 μm, the electric resistivity maybecome too high. If the average pore diameter of each electrode portion21 is more than 45 μm, the strength of each electrode portion 21 may beweakened and each electrode portion 21 may tend to be broken. Theaverage pore size is a value measured with a mercury porosimeter.

When each electrode portion 21 is mainly based on the “silicon-siliconcarbide composite material”, silicon carbide particles contained in eachelectrode portion 21 preferably have an average particle diameter offrom 10 to 60 μm, and more preferably 20 to 60 μm. The average particlediameter of the silicon carbide particles contained in the electrodeportion 21 in such a range can allow the electric resistivity of theelectrode portion 21 to be controlled within a range of from 0.1 to 100Ωcm. If the average pore diameter of the silicon carbide particlescontained in the electrode portion 21 is less than 10 μm, the electricresistivity of each electrode portion 21 may become too high. If theaverage pore diameter of the silicon carbide particles contained in theelectrode portion 21 is more than 60 μm, the strength of each electrodeportion 21 may be weakened and each electrode portion 21 may tend to bebroken. The average particle diameter of the silicon carbide particlescontained in the electrode portion 21 is a value measured by a laserdiffraction method.

When each electrode portion 21 is mainly based on the “silicon-siliconcarbide composite material”, a ratio of a mass of silicon contained inthe electrode portions 21 to “the total of the respective masses ofsilicon carbide particles and silicon” contained in the electrodeportions 21 is preferably in a range of from 20 to 40% by mass. Theratio of the mass of silicon to “the total of the respective masses ofsilicon carbide particles and silicon” contained in the electrodeportions 21 is more preferably from 25 to 35% by mass. By such a rangeof the ratio of the mass of silicon to “the total of the respectivemasses of silicon carbide particles and silicon” contained in theelectrode portions 21, the electric resistivity of the electrodeportions 21 can be in a range of from 0.1 to 100 Ωcm. If the ratio ofthe mass of silicon to “the total of the respective masses of siliconcarbide particles and silicon” contained in the electrode portions 21 isless than 20% by mass, the electric resistivity may become too high, andif it is more than 40% by mass, deformation may tend to occur during theproduction.

The honeycomb structure 100 according to the present embodimentpreferably has an isostatic strength of 1 MPa or more, and morepreferably 3 MPa or more. The isostatic strength is preferably as highas possible, but an upper limit will be about 6 MPa, in view of thematerial, structure and the like of the honeycomb structure 100. If theisostatic strength is less than 1 MPa, the honeycomb structure may beeasily broken when used as a catalyst support or the like. The isostaticstrength is a value measured by applying a hydrostatic pressure inwater.

(3. Production Method)

Production of the honeycomb structure portion can be carried out inaccordance with a method for making a honeycomb structure portion in aknown method for producing a honeycomb structure portion. For example,first, a forming material is prepared by adding metallic silicon powder(metallic silicon), a binder, a surfactant(s), a pore former, water, andthe like to silicon carbide powder (silicon carbide). It is preferablethat a mass of metallic silicon is from 10 to 40% by mass relative tothe total of mass of silicon carbide powder and mass of metallicsilicon. The average particle diameter of the silicon carbide particlesin the silicon carbide powder is preferably from 3 to 50 μm, and morepreferably from 3 to 40 μm. The average particle diameter of themetallic silicon particles in the metallic silicon powder is preferablyfrom 2 to 35 μm. The average particle diameter of each of the siliconcarbide particles and the metallic silicon particles refers to anarithmetic average diameter on volume basis when frequency distributionof the particle size is measured by the laser diffraction method. Thesilicon carbide particles are fine particles of silicon carbide formingthe silicon carbide powder, and the metallic silicon particles are fineparticles of metallic silicon forming the metallic silicon powder. Itshould be noted that this is formulation for forming raw materials inthe case where the material of the honeycomb structure portion is thesilicon-silicon carbide composite material. In the case where thematerial of the honeycomb structure portion is silicon carbide, nometallic silicon is added.

Examples of the binder include methyl cellulose, hydroxypropyl methylcellulose, hydroxypropoxyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl alcohol and the like. Among these, itis preferable to use methyl cellulose in combination withhydroxypropoxyl cellulose. The content of the binder is preferably from2.0 to 10.0 parts by mass when the total mass of the silicon carbidepowder and the metallic silicon powder is 100 parts by mass.

The content of water is preferably from 20 to 60 parts by mass when thetotal mass of the silicon carbide powder and the metallic silicon powderis 100 parts by mass.

The surfactant that can be used includes ethylene glycol, dextrin, fattyacid soaps, polyalcohol and the like. These may be used alone or incombination of two or more. The content of the surfactant is preferablyfrom 0.1 to 2.0 parts by mass when the total mass of the silicon carbidepowder and the metallic silicon powder is 100 parts by mass.

The pore former is not particularly limited as long as the pore formeritself forms pores after firing, including, for example, graphite,starch, foamed resins, water absorbing resins, silica gel and the like.The content of the pore former is preferably from 0.5 to 10.0 parts bymass when the total mass of the silicon carbide powder and the metallicsilicon powder is 100 parts by mass. An average particle diameter of thepore former is preferably from 10 to 30 μm. If it is less than 10 μm,pores may not be sufficiently formed. If it is more than 30 μm, a diemay be clogged with the pore former during forming. The average particlesize of the pore former refers to an arithmetic average diameter onvolume basis when frequency distribution of the particle size ismeasured by the laser diffraction method. When the pore former is thewater absorbing resin, the average particle diameter of the pore formeris an average particle diameter after water absorption.

Then, the resulting forming raw materials are kneaded to form a greenbody, and the green body is then extruded to prepare a honeycombstructure portion. In extrusion molding, a die having a desired overallshape, cell shape, partition wall thickness, cell density and the likecan be used. Preferably, the resulting honeycomb structure portion isdried. A drying method is not particularly limited, and examples of themethod include electromagnetic wave heating methods such as microwaveheating and drying, and high frequency dielectric heating and drying,and external heating methods such as hot air drying and superheatedsteam drying. Among them, from the viewpoint that the entire formedproduct can be quickly and uniformly dried so as not to cause cracks, acertain amount of moisture is dried by the electromagnetic wave heatingmethod, and the remaining moisture is dried by the external heatingmethod. The drying is preferably carried out under conditions wheremoisture of from 30 to 99% by mass is removed relative to the moistureamount before drying by the electromagnetic wave heating method, and themoisture is then reduced to 3% by mass or less by the external heatingmethod. The electromagnetic wave heating method is preferably dielectricheating drying, and the external heating method is preferably hot airdrying. A drying temperature is preferably from 50 to 100° C.

When the length in the central axis direction of the honeycomb structureportion is not the desired length, both the end faces of the honeycombstructure portion can be cut to the desired length. A non-limitingcutting method includes a method that utilizes a circular saw cuttingmachine or the like.

The honeycomb dried body is then fired to prepare a honeycomb firedbody. In the firing, for example, the honeycomb dried body is fired inan Ar atmosphere at 1400° C. for 3 hours. In the firing, the electricresistivity distribution of the present invention is easily achieved by,for example, placing a screen made of the same material between sheaths(which refer to enclosures used in the firing) for a site to be highresistance relative to a site to be low resistance, and then firing thehoneycomb dried body in an Ar atmosphere containing 1.0 vol % of N₂ at1400° C. for 3 hr.

It should be noted that a means for achieving the electric resistivitydistribution of the present invention is not particularly limited, andthe electric resistivity distribution of the present invention can beachieved, even by changing factors affecting the electric resistivity,such as the material of the honeycomb structure portion and the wallthickness, in addition to the above means.

Before firing, calcination may preferably be carried out in order toremove the binder and the like. The calcination is preferably performedin an air atmosphere at a temperature of from 400 to 500° C. for 0.5 to20 hours. The methods of calcination and firing are not limited, andthey may be carried out using an electric furnace, a gas furnace or thelike. The firing can be preferably carried out in an inert atmospheresuch as nitrogen or argon at a temperature of from 1300 to 1500° C. for1 to 20 hours. After firing, an oxygenation treatment is preferablycarried out at a temperature of from 1000 to 1250° C. for 1 to 10 hoursin order to improve durability.

The electrode portions are then formed on the honeycomb dried body. Itis preferable to prepare an electrode portion-forming raw material forforming the electrode portions. When each electrode portion is mainlybased on the “silicon-silicon carbide composite material”, the electrodeportion-forming material is preferably formed by adding certainadditives to silicon carbide powder and silicon powder and kneading themixture.

More particularly, it is preferable to add metallic silicon powder(metallic silicon), a binder, a surfactant(s), a pore former, water andthe like to silicon carbide powder (silicon carbide), and knead them toprepare an electrode portion-forming material. When the total mass ofsilicon carbide powder and metallic silicon is 100 parts by mass, themass of metallic silicon is preferably from 20 to 40 parts by mass. Theaverage particle diameter of the silicon carbide particles in thesilicon carbide powder is preferably from 10 to 60 μm. The averageparticle size of the metallic silicon powder (metallic silicon) ispreferably from 2 to 20 μm. If the average particle diameter of themetallic silicon powder (metallic silicon) is less than 2 μm, theelectric resistivity may become too low. If the average particlediameter of the metallic silicon powder (metallic silicon) is more than20 μm, the electric resistivity may become too high. The averageparticle diameter of each of silicon carbide particles and metallicsilicon (metallic silicon particles) is a value measured by the laserdiffraction method. The silicon carbide particles are fine particles ofsilicon carbide forming the silicon carbide powder, and the metallicsilicon particles are fine particles of metallic silicon forming themetallic silicon powder.

Examples of the binder include methyl cellulose, hydroxypropyl methylcellulose, hydroxypropoxyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl alcohol and the like. Among these, itis preferable to use methyl cellulose in combination withhydroxypropoxyl cellulose. The content of the binder is preferably from0.1 to 5.0 parts by mass when the total mass of the silicon carbidepowder and the metallic silicon powder is 100 parts by mass.

The content of water is preferably from 15 to 60 parts by mass when thetotal mass of the silicon carbide powder and the metallic silicon powderis 100 parts by mass.

The surfactant that can be used includes ethylene glycol, dextrin, fattyacid soaps, polyalcohol and the like. These may be used alone or incombination of two or more. The content of the surfactant is preferablyfrom 0.1 to 2.0 parts by mass when the total mass of the silicon carbidepowder and the metallic silicon powder is 100 parts by mass.

The pore former is not particularly limited as long as the pore formeritself forms pores after firing, including, for example, graphite,starch, foamed resins, water absorbing resins, silica gel and the like.The content of the pore former is preferably from 0.1 to 5.0 parts bymass when the total mass of the silicon carbide powder and the metallicsilicon powder is 100 parts by mass. An average particle diameter of thepore former is preferably from 10 to 30 μm. If it is less than 10 μm,pores may not be sufficiently formed. If it is more than 30 μm, largerpores may tend to occur, thereby causing a decrease in strength. Theaverage particle size of the pore former is a value measured by thelaser diffraction method.

Preferably, a mixture obtained by mixing silicon carbide powder (siliconcarbide), metallic silicon (metallic silicon powder), a binder, asurfactant(s), a pore former, water and the like is then kneaded toobtain an electrode portion-forming raw material in the form of paste orslurry. The kneading method is not particularly limited, and forexample, a vertical type stirrer may be used.

Preferably, the resulting electrode portion-forming material is thenapplied to a side surface of the honeycomb fired body. The method ofapplying the electrode portion-forming material to the side surface ofthe honeycomb fired body is not particularly limited, but for example, aprinting method can be used. Further, the electrode portion-formingmaterial is preferably applied to the side surface of the honeycombfired body so as to have the shape of each electrode portion in thehoneycomb structure according to the present invention. Each electrodeportion can have a desired thickness by adjusting the thickness at thetime of applying the electrode portion-forming raw material. Thus, theelectrode portions can be formed only by applying the electrodeportion-forming material to the side surface of the honeycomb fired bodyand drying and firing it, so that the electrode portions can be veryeasily formed.

Subsequently, the electrode portion-forming material applied to the sidesurface of the honeycomb fired body is preferably dried to form unfiredelectrodes and to prepare a honeycomb fired body with unfiredelectrodes. The drying condition is preferably from 50 to 100° C.

The honeycomb fired body with unfired electrodes is then fired toprepare a honeycomb structure. In this case, the unfired electrodes aremainly fired. Before firing, calcination is preferably carried out inorder to remove the binder and the like. The calcination is preferablyperformed in an air atmosphere at a temperature of from 400 to 500° C.for 0.5 to 20 hours. The methods of calcination and firing are notlimited, and they may be carried out using an electric furnace, a gasfurnace or the like. The firing can be preferably carried out in aninert atmosphere such as nitrogen or argon at a temperature of from 1400to 1500° C. for 1 to 20 hours. After firing, an oxygenation treatment ispreferably carried out at a temperature of from 1200 to 1350° C. for 1to 10 hours in order to improve durability.

EXAMPLES

Hereinafter, while the present invention will be more specificallydescribed with reference to Examples, the present invention is notlimited to these Examples.

Metallic silicon (Si) powder was used as a ceramic raw material. To theceramic raw material were added hydroxypropyl methyl cellulose as abinder, a water absorbing resin as a pore former, and water to form aforming raw material. The forming raw material was then kneaded by meansof a vacuum green body kneader to prepare a pillar shaped green body.The content of the binder was 7 parts by mass based on 100 parts by massof metallic silicon (Si) powder. The content of the pore former was 3parts by mass based on 100 parts by mass of metallic silicon (Si)powder. The content of water was 42 parts by mass based on 100 parts bymass of metallic silicon (Si) powder. The average particle diameter ofmetallic silicon (Si) powder was 6 μm. The average particle diameter ofthe pore former was 20 μm. The average particle diameter of each of themetallic silicon (Si) and the pore former is a value measured by thelaser diffraction method.

The resulting pillar shaped green body was formed using an extruder toobtain a honeycomb formed body having a diameter of 80 mm. The resultinghoneycomb formed body was subjected to high-frequency dielectric heatingand drying and then dried at 120° C. for 2 hours using a hot air drier,and a predetermined amount of both end faces were cut to prepare ahoneycomb dried body having a length of 75 mm.

Subsequently, the honeycomb dried body was degreased (calcined) and thenfired. The conditions for the firing were 1370° C. for 3 hr forComparative Examples 1 and 2, and 1390° C. for 3 hr for Examples 1 and2. For Examples 1 and 2, the firing was performed in an Ar atmospherecontaining 1.0 vol % of N₂ at 1390° C. for 3 hours.

The honeycomb body after the firing was further oxidized to obtain ahoneycomb fired body. Degreasing was carried out at 550° C. for 3 hours.An oxidation treatment was carried out at 1300° C. for 1 hour.

Then, to metallic silicon (Si) powder were added hydroxypropyl methylcellulose as a binder, glycerin as a humectant, and a surfactant as adispersant, and water and mixed together. The mixture was kneaded toprepare an electrode portion-forming raw material. The content of thebinder was 0.5 parts by mass based on 100 parts by mass of metallicsilicon (Si) powder, and the content of the glycerin was 10 parts bymass based on 100 parts by mass of metallic silicon (Si) powder, and thecontent of the surfactant was 0.3 parts by mass based on 100 parts bymass of metallic silicon (Si) powder, and the content of water was 42parts by mass based on 100 parts by mass of metallic silicon (Si)powder. The average particle diameter of metallic silicon (Si) powderwas 6 μm. The average particle diameter of metallic silicon (Si) is avalue measured by the laser diffraction method. The kneading was carriedout by means of a vertical stirrer.

The electrode portion-forming raw material was then applied onto theside surface of the honeycomb fired body, in a strip shape so as toextend between the both end faces of the honeycomb fired body, such thata thickness was 1.5 mm, and “0.5 times of a central angle in the crosssection orthogonal to the extending direction of the cell was 50°”. Theelectrode portion-forming material was applied to two positions on theside surface of the honeycomb fired body. Then, in the cross sectionorthogonal to the extending direction of the cell, one of the twoportions coated with the electrode portion-forming material was disposedon a side opposite to the other, across the center of the honeycombfired body.

The electrode portion-forming raw material applied to the honeycombfired body was then dried to obtain a honeycomb fired body with unfiredelectrodes. The drying temperature was 70° C.

Subsequently, the honeycomb dried body was degreased (calcined), firedand further oxidized to obtain a honeycomb structure. Degreasing wascarried out at 550° C. for 3 hours. The firing was performed in an Aratmosphere at 1450° C. for 2 hours. An oxidation treatment was carriedout at 1300° C. for 1 hour.

The average pore diameter (pore diameter) of the partition walls of theresulting honeycomb structure was 8.6 μm and the porosity was 45%. Theaverage pore diameter and the porosity are values measured by themercury porosimeter. Further, the thickness of the partition wall of thehoneycomb structure was 90 μm, and the cell density was 90 cells/cm².Furthermore, each end face of the honeycomb structure was circular witha diameter of 93 mm, and the length of the honeycomb structure in theextending direction of the cell was 75 mm. In addition, the resultinghoneycomb structure had an isostatic strength of 2.5 MPa. The isostaticstrength is a fracture strength measured by applying hydrostaticpressure in water. The central angles of the two electrode portions ofthe honeycomb structure in the cross section orthogonal to the extendingdirection of the cell are shown in Table 1.

Further, the electric resistivity of each of the electrode portions ofthe honeycomb structures of Comparative Examples and Examples wasmeasured at room temperature (25° C.), and it was 1.0 Ω·cm for all ofComparative Examples and Examples.

An electrical current test was carried out for each honeycomb structureobtained by the above procedures. In the electrical current test,temperatures of the end regions and the central region were measured asfollows, after 20 seconds when terminals were connected to a pair ofterminal connection portions and a voltage was applied with an inputpower of 1.5 kW. In the cross section orthogonal to the extendingdirection of the cell of the honeycomb structure portion, a temperatureeach of two points on a straight line L and at a distance of ⅕ L fromthe outer peripheral wall of each honeycomb structure portion 10 wasmeasured, and an average value thereof was defined as a temperature ofthe end portion region. A temperatures at each of two points at adistance of further ⅕ L from each of the above two points on thestraight line L (i.e., two points at a distance of ⅖ L from the outerperipheral wall of the honeycomb structure portion 10) and a temperatureat each of two points at a distance of ⅕ L from the outer peripheralwall of each honeycomb structure 10 on a straight line passing through Oand perpendicular to the straight line L were measured, and an averagevalue of the temperatures of these four points was determined to be atemperature of the central region.

The electric resistivity of each of the end regions and the centralregion of each honeycomb structure portion 10 of Comparative Examplesand Examples was measured as follows. In the cross section orthogonal tothe extending direction of the cell of the honeycomb structure portion,as shown in FIG. 6, the electric resistivity of each of the two pointson the straight line L at a distance of ⅕ L from the outer peripheralwall of each honeycomb structure portion 10 was measured, and an averagevalue thereof was defined as the electric resistivity of the end region.The electric resistivity of each of two points at a distance of further⅕ L from each of the above two points on the straight line L (that is,two points at a distance of ⅖ L from the outer peripheral wall of eachhoneycomb structure portion 10) and the electric resistivity of each oftwo points at a distance of ⅕ L from the outer peripheral wall of thehoneycomb structure portion 10 on a straight line passing through thecenter O of the honeycomb structure portion 10 and perpendicular to thestraight line L were measured, and an average value of the electricresistivity values of these four points was determined to be theelectric resistivity of the central region. The electric resistivity wasmeasured by a four-terminal method using a multimeter. The measurementresults are shown in Table 1.

TABLE 1 Uniform Heat Central Angle Electric Resistivity (Ω · cm)Temperature (° C.) Generation of Electrode End Central End Central(Maximum Temp. − Portion (°) Region A Region B Region Region MinimumTemp.) Comparative 90 100 100 130 234 104 Example 1 Comparative 90 80100 146 219 73 Example 2 Example 1 90 80 100 156 207 51 Example 2 90 20100 183 204 21

DESCRIPTION OF REFERENCE NUMERALS

-   100 . . . honeycomb structure-   10 . . . honeycomb structure portion-   11 . . . partition wall-   12 . . . cell-   13, 14 . . . both end faces of the honeycomb structure portion-   21 . . . electrode portion

1. A honeycomb structure, comprising: a pillar shaped honeycomb structure portion having: porous partition walls extending through the honeycomb structure from an inflow end face to an outflow end face to define a plurality of cells forming a through channel; an outer peripheral wall located at the outermost periphery; and a pair of electrode portions disposed on a side surface of the honeycomb structure portion; wherein each of the pair of electrode portions is formed in a strip shape extending in an extending direction of the cells of the honeycomb structure portion; wherein, in a cross section orthogonal to the extending direction of the cells, one electrode portion of the pair of electrode portions is disposed on a side opposed to the other electrode portion across a center of the honeycomb structure portion; wherein the honeycomb structure portion consists: end regions near the pair of electrode portions; and a central region that is a center region excluding the end regions; and wherein an average electric resistivity A of a material forming the end regions is lower than an average electric resistivity B of a material forming the central region.
 2. The honeycomb structure according to claim 1, wherein the A and the B satisfy a relationship: ⅕≤A/B≤⅘.
 3. The honeycomb structure according to claim 1, wherein the honeycomb structure portion is mainly based on a silicon-silicon carbide composite material or silicon carbide.
 4. The honeycomb structure according to claim 1, wherein the honeycomb structure portion has an electric resistivity of from 0.1 to 100 Ωcm and the electrode portions have an electric resistivity of from 0.001 to 1.0 Ωcm.
 5. The honeycomb structure according to claim 1, wherein each of the electrode portions has a central angle of from 60 to 120°. 